1. Field of the Invention
This invention relates to amplifiers in general and, more particularly, to current-mode feedback amplifiers.
2. Description of the Prior Art
Current-mode feedback operational amplifiers are being adopted for high performance applications instead of voltage-mode feedback operational amplifiers because of the superior gain-bandwidth product and faster slew rate, as well as other advantages, of current-mode feedback amplifiers.
Current-mode feedback operational amplifiers are distinguished from voltage-mode feedback amplifiers primarily by the input stage of the amplifier having one relatively low impedance input (typically the inverting input) and relatively high impedance input (typically the non-inverting input.) Prior art exemplary input stages for a current-mode feedback amplifier are shown in U.S. Pat. No. 4,780,689. Unfortunately, the low impedance input of these prior art stages may not be sufficiently low for very high performance applications. As a consequence, the higher impedance reduces the possible gain-bandwidth product of the amplifier. Further, the prior art input stages suffer from a relatively low input dynamic range which, when used with a ground-referenced +5 volt (or lower) only supply, can be severely limiting. The amplifier should also have low input offset voltages.
Another application of the input stage for a current-mode feedback amplifier is as a unity gain buffer having a high input impedance and a low output impedance with a minimal voltage offset from input to output. Typically, the output of the buffer is the inverting low impedance input of the input stage of the current-mode feedback amplifier. The above discussed problems with impedance and the dynamic range limitations are substantially the same with the unity gain buffer as with the input stage of the current-mode feedback amplifier.
Thus, it is desirable to provide an input stage for a current-mode feedback amplifier or a unity gain buffer that has low offset, low output impedance, and large input dynamic range.